fork failing when calling initialize_dup

initialize_dup has been made private as of ruby 2.0.0. I have
corrected it to call the publicly available dup method

fix ace editor Syntax Highlighter mode load 404

add proper code/command formatting where missing in CONTRIBUTING.md

link to RSpec. related to #3810

link to spinach in the README. Closes #3810

Internally public projects

updated fork feature to use gitlab-shell for v5 of gitlab

fix http push 401 and 500 with some git clients

See https://github.com/gitlabhq/gitlabhq/pull/3597#discussion-diff-4014638

When asking to fork a project and a project with the same name
already exists (likely from a previous fork), the recovery from
the fork failure would inadvertantly delete the repo of the
existing destination project.

In the public area, project names are shown as static text for
non-logged in users, while logged-in users are given project
names as links they can follow to the project's page.

Assignees are now sorted alphabetically.

[BUGFIX] User suggest at team_members/new not working with relative_url_root

Takes into account 98bea4b1 and 3b88636d.

Public projects listed in the public section will be linked to the
actual project's page. Public projects now give any user Guest
permissions to the project, allowing them to download the code, read
and create issues, and view anything else in the project's pages.

Ample access tests have been added to the project_access_spec to
verify correct permissions and behavior on public projects.
- Visitors to the site who are not logged in still cannot view the
  project's pages.
- Logged-in users visiting a public project where they are not a team
  member can create issues, but not snippets. They can view the projects
  code, issues, merge requests, etc, just as if they were a Guest member
  of the project.
- Since this is a public project, the user is also granted :download_code
  permissions, a permission normally reserved for Reporters, since they
  can clone the repo anyways and browse commits and branches locally.

Swapped out placeholder text on teams admin page

Changed "simple Teams description" placeholder subtitle to "allow you
to organize groups of people that have a common focus. Use teams to
simplify the process of assigning roles to groups of people."